The investigated assay requires the measurement of three host-related, blood-based, protein biomarkers that are being integrated using a logistic regression formula into a single score. Based on this score, each patient is classified into one of three categories: (i) bacterial immune response (i.e., pure bacterial infections and mixed bacterial and viral co-infection), (ii) viral immune response, and (iii) marginal immune response (inconclusive or non-infectious). It is estimated that 10-20% of the infected patients will have a marginal immune response. A composite reference standard will be used in order to determine the diagnosis of each patient. Specifically, all the clinical, radiological, microbiological and laboratory data of each patient, will be recorded in a dedicated eCRF. Based on this data, the diagnosis of each patient will be determined by a panel of three independent pediatricians. Each pediatrician will be blinded to the diagnosis of his peers and to the assay results. In the current study, unanimous agreement between the experts ("consensus agreement") will be considered as the true diagnosis for the purpose of computing the assay performance.